Liquid crystal displays ("LCD's"), in which the electro-optically active element comprises liquid crystals, are well known in the art. One type of LCD employs an encapsulated liquid crystal material in which a liquid crystal composition is encapsulated or dispersed in many tiny volumes or droplets in a containment medium such as a polymer. When a voltage corresponding to a sufficiently strong electric field is applied across the encapsulated liquid crystal material (the "field-on" condition), the alignment of the liquid crystal molecules is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystal molecules is random and/or influenced by the liquid crystal-matrix interface, so that the material scatters and/or absorbs incident light.
One method of making encapsulated liquid crystal material is to form an emulsion of the liquid crystal composition in the containment medium, in the presence of a carrier medium such as water. The emulsion is coated onto a substrate and the carrier medium is allowed to evaporate. In this method, the containment medium is normally in a fully polymerized form when the emulsion is formed. Illustrative disclosures include Fergason, U.S. Pat. No. 4,435,047 (1984); Fergason, U.S. Pat. No. 4,616,903 (1986); Fergason et al., U.S. Pat. No. 4,950,052 (1990); Pearlman, U.S. Pat. No. 4,992,201 (1991); and Ma et al., U.S. Pat. No. 5,506,898 (1991).
Another method of making encapsulated liquid crystal material is generally referred to as the phase separation method: an initially homogeneous combination of the liquid crystal composition and containment medium (or precursor thereof) thereof) is prepared and then phase separation of the liquid crystal composition is induced by altering the ambient conditions, to form the encapsulated liquid crystal material. In the variation known as temperature-induced phase separation, the liquid crystal composition is dissolved in the containment medium at an elevated temperature. Upon cooling, the liquid crystal composition becomes less soluble and phase separates. In the variation known as solvent-induced phase separation, a solution of the liquid crystal composition and containment medium in a solvent is prepared. As the solvent is allowed to evaporate, the solubility of the liquid crystal composition decreases, and phase separation occurs. In the variation known as polymerization-induced phase separation, a solution of the liquid crystal composition in a precursor of the containment medium is formed. The precursor is allowed to polymerize to form the final containment medium. As polymerization proceeds, the solubility of the liquid crystal composition decreases and phase separation occurs. Illustrative disclosures include Wu et al., U.S. Pat. No. 4,671,618(1987); West et al., U.S. Pat. No. 4,673,255 (1987); West et al., U.S. Pat. No. 4,685,771 (1987); Doane et al., U.S. Pat. No. 4,688,900 (1987); Vaz et al., U.S. Pat. No. 4,728,547 (1988); Doane et al., U.S. Pat. No. 4,994,204 (1991); West, U.S. Pat. No. 5,004,323 (1991); and Arai et al., EP 0,313,053 A2 (1989).
Other disclosures of interest include Churchill et al., U.S. Pat. No. 3,600,060 (1971); Cartmell et al., U.S. Pat. No. 3,720,623 (1973); Benton et al., U.S. Pat. No. 3,872,050 (1975); Taylor, U.S. Pat. No. 3,935,337 (1976); Taylor, U.S. Pat. No. 4,101,207 (1978); Benton et al., U.S. Pat. No. 4,182,700 (1980); and Benton et al., U.S. Pat. No. 4,246,302 (1981).
The prior art methods suffer from a number of limitations. The requirement for removal of the carrier medium or solvent precludes the preparation of LCD's by "cell-fill" when an emulsion or solvent-induced phase separation process is used. (In a cell-fill technique, the encapsulated liquid crystal material is introduced (e.g., by capillary or vacuum action) into the narrow gap of a preformed LCD cell.) Rather, a sequence of coating, solvent removal, and lamination steps must be used.
Polymerization- and temperature-induced phase separation are in theory compatible with the cell-fill technique, but once the phase separation process has begun, it is difficult to stop or control. The size and size distribution of the liquid crystal droplets or volumes has a marked effect in the performance characteristics of the resulting LCD. It has been remarked that control of size distribution is poor in LCD's prepared by the phase separation process (Vaz et al., SPIE Vol. 1455, Liquid Crystal Devices and Materials (1991), pp. 110-121).
This invention provides a novel method of making encapsulated liquid crystal materials solving the aforementioned limitations.